The invention relates to a piston pump which is intended in particular for use in an electrohydraulic vehicle brake system, where it serves the purpose of brake pressure buildup. It can also be used in some other, in particular a slip-controlled, vehicle brake system.
One such piston pump is known from German Patent Disclosure DE 44 25 402 A1. The known piston pump has a pump housing, in which a piston is guided axially displaceably. The piston can be guided either directly in the pump housing or in a bush inserted into the pump housing. For driving the piston, the known piston pump has an eccentric element, which can be driven to rotate by an electric motor and which drives the piston to a stroke motion that reciprocates in the axial direction. The stroke motion includes a supply stroke, in which a volume of a positive displacement chamber of the piston pump is reduced in size and fluid to be pumped is positively displaced out of the piston pump, and a return stroke, in which fluid flows into the positive displacement chamber. A turning point of the piston at the end of the supply stroke and the beginning of the return stroke will, for the purposes of the present invention, be called top dead center. In a manner known per se, the stroke motion of the piston brings about pumping of fluid that is to be pumped.
For controlling a fluid flow, the known piston pump has an inlet valve, embodied as a spring-loaded check valve, whose valve closing spring is braced in stationary fashion in the pump housing and presses a valve closing body of the inlet valve against a valve seat embodied on one face end of the piston. Because of the stationary bracing of the valve closing spring in the pump housing, the length of the valve closing spring changes during the piston stroke. The change in length of the valve closing spring causes a change in a spring force exerted on the valve closing body by the valve closing spring. This spring force, which presses the valve closing body against the valve seat, is the greatest at the end of the supply stroke and at the beginning of the return stroke, or in other words at top dead center, and therefore precisely whenever the inlet valve is supposed to open at the beginning of the return stroke of the piston.
To improve an opening performance of the inlet valve in the known piston pump, its valve closing body is provided with a pin, which is in frictional engagement with a bush mounted in stationary fashion in the pump housing. The frictional force exerted on the valve closing body is always oriented counter to a motion; at the beginning of the return stroke of the piston, this force acts on the valve closing body in an opening direction. As a result, the frictional force reinforces the opening of the inlet valve at the onset of the return stroke. The frictional force exerted on the valve closing body is selected such that it is less than a minimum spring force of the valve closing spring at maximum length of the valve closing spring, or in other words at bottom dead center of the piston, so that at bottom dead center the inlet valve will be closed by the valve closing spring. As a result of this limitation of the frictional force, its effect that reinforces the opening of the inlet valve is limited, since at top dead center of the piston, or in other words precisely when the inlet valve is supposed to open, the spring force of the valve closing spring is greatest. Compulsory opening of the inlet valve by the frictional force exerted on its valve closing body by the bush, which is stationary in the pump housing, fails to occur.
For opening the inlet valve in the known piston pump, a pressure difference must prevail at the valve closing body of the inlet valve, and this difference, reinforced by the frictional force exerted on the valve closing body, lifts the valve closing body from the valve seat, counter to the spring force of the valve closing spring which is at maximum at top dead center. This pressure difference is brought about during the return piston stroke, which reduces the volume of the positive displacement chamber of the piston pump in the supply stroke and increases it in the return stroke. A disadvantage is that to effect the pressure difference at the inlet valve required for opening the inlet valve, the piston must travel some distance from top dead center in the return stroke direction. Especially at low temperatures, below freezing, the piston travel required to open the inlet valve is lengthened because of the increasing viscosity of the fluid to be pumped, which for instance is a brake fluid. The delay in opening the inlet valve is amplified still more by outgassing effects, which speed up at low temperatures. The outgassing effects cause an increase in the volume of the fluid in the positive displacement chamber of the piston pump, when the volume of the positive displacement chamber increases in the return stroke of the piston. Because of the increase in volume of the fluid caused by the outgassing effects, the buildup of the pressure difference at the inlet valve in the return stroke of the piston is slower, and the distance traveled by the piston until the inlet valve opens is lengthened. The result is an impairment of the pumping capacity of the piston pump.
The piston pump according to the invention has a valve opening device which compulsorily opens the inlet valve at the beginning of the return stroke of the piston. The inlet valve of the piston pump of the invention is opened independently of a pressure difference applied to it and also counter to the force of any valve closing spring that may be present. As a result, from the onset of the return stroke of the piston, fluid to be pumped flows into the positive displacement chamber of the piston pump, and especially at low temperatures as well, the entire return stroke of the piston is available and is used for the inflow of the fluid to be pumped into the positive displacement chamber. The piston pump of the invention has the advantage of an improved pumping capacity and enhanced efficiency, especially at low temperatures.
The piston pump of the invention furthermore has a valve stroke limiter, which limits an opening stroke of a valve closing body of the inlet valve. During the return stroke of the piston, the valve stroke limiter acts as a slaving means for the valve closing body of the inlet valve, which body slaves the valve closing body to the piston in the return stroke of the piston in the fully open position. This has the advantage that a valve closing time at the end of the return stroke is short.
To improve a closing performance of the inlet valve, the piston pump in one embodiment of the invention has a valve closing spring for the inlet valve. This spring is preferably mounted on the piston, so that the spring force of the valve closing spring is independent of a piston position.
According to one feature of the invention it is provided that the inlet valve mounted on the piston, before reaching top dead center, enters at the end of the supply stroke of the piston into a force-releasable engagement with the valve opening device, which is mounted in stationary fashion in the pump housing. The force-releasable engagement can also exist with a part of the valve opening device that is stationary in the pump housing, while conversely another part of the valve opening device, which enters into engagement with the stationary part, is connected to the valve closing body and is slaved to it. The force-releasable engagement is nonpositive or frictional and/or positive. After top dead center has been overcome, the valve opening device of the valve closing body restrains the inlet valve and thereby opens the inlet valve. The engagement force exerted on the valve closing body by the valve opening device is so great that it opens the inlet valve even counter to the force of any valve closing spring of the inlet valve that may be present. The valve opening device in this embodiment of the invention is operative only when the piston is located near top dead center, or in other words at the end of the supply stroke and the beginning of the return stroke.
After the opening of the inlet valve, the valve stroke limiter puts the valve closing body of the inlet valve out of engagement with the valve opening device by means of the return stroke motion of the piston, so that the inlet valve acts like a conventional check valve and closes automatically at the end of the return stroke.
According to another feature of the invention, the valve closing body of the inlet valve remains in frictional engagement with the valve opening device during the entire piston motion. As a result, the inlet valve is opened compulsorily at the beginning of the piston return stroke and kept open during the return stroke. At the beginning of the supply stroke, the inlet valve is closed again by the valve opening device and is kept closed during the supply stroke.
The piston of the piston pump of the invention is preferably embodied as a stepped piston. The stepped piston is guided, on its end toward the positive displacement chamber, over a larger diameter than on its end remote from the positive displacement chamber. This creates an annular chamber, which surrounds the piston on its smaller diameter and whose volume increases in the supply stroke of the piston and decreases in the return stroke. The increase in size of the annular chamber in the supply stroke causes an aspiration of fluid to be pumped. In the return stroke, while the volume of the annular chamber is indeed decreased, nevertheless the volume of the positive displacement chamber of the piston pump increases to a greater extent, so that fluid to be pumped overflows from the annular chamber into the positive displacement chamber. The use of a stepped piston in the piston pump of the invention has the advantage that by means of the inlet valve that opens compulsorily at the beginning of the return stroke, fluid to be pumped overflows from the annular chamber into the positive displacement chamber, and the overflow is reinforced by inertia on the part of fluid aspirated into the annular chamber during the supply stroke and accelerated in the process. If the inlet valve is not compulsorily opened, then the inlet valve at the beginning of the return stroke of the piston causes a counterpressure on an inflow side of the inlet valve, which brakes the fluid to be pumped, which is accelerated during the supply stroke in the inlet of the piston pump, and counteracts an overflow of fluid, aspirated into the annular chamber during the supply stroke, into the positive displacement chamber of the piston pump.
The piston pump according to the invention is intended in particular as a pump in a brake system of a vehicle and is used in controlling the pressure in wheel brake cylinders. Depending on the type of brake system, the abbreviations ABS (anti-lock brake system), TCS (traction control system), ESP (electronic stability program) and EHB (electrohydraulic brake system) are used for such brake systems. In the brake system, the pump serves for instance to return brake fluid from a wheel brake cylinder or a plurality of wheel brake cylinders to a master cylinder (ABS) and/or to pump brake fluid out of a supply container into a wheel brake cylinder or a plurality of wheel brake cylinders (TCS or ESP or EHB). For instance, the pump is needed in a brake system with wheel slip control (ABS or TCS) and/or a brake system serving as a steering aid (ESP) and/or an electrohydraulic brake system (EHB). With wheel slip control (ABS or TCS), locking of the wheels of the vehicle during a braking event involving strong pressure on the brake pedal (ABS) and/or spinning of the driven wheels of the vehicle in the event of strong pressure on the gas pedal (TCS) can for instance be prevented. In a brake system serving as a steering aid (ESP), a brake pressure is built up in one or more wheel brake cylinders independently of an actuation of the brake pedal or gas pedal, for instance to prevent the vehicle from breaking out of the lane desired by the driver. The pump can also be used in an electrohydraulic brake system (EHB), in which the pump pumps the brake fluid into the wheel brake cylinder or wheel brake cylinders if an electric brake pedal sensor detects an actuation of the brake pedal, or in which the pump is used to fill a reservoir of the brake system.